There's no outward bruises, scars or bleeding, just the sort of symptoms (headaches, fatigue, short-memory loss) that could easily be ignored.

But in the world of youth and high school sports, there are few injuries that have more potential for long-term damage because of second impact syndrome (SIS).

SIS is a condition that causes the brain to swell after a person suffers a second concussion before symptoms from an earlier one have subsided.

Believed to be more common with adolescent athletes whose brains are still developing, it can prove fatal or lead to life-long disabilities.

Dr. Gerry Gioia -- chief of pediatric neuropsychology at Children's National Medical Center in Washington, D.C. -- is one of many in the medical community who have tried to educate the public about the danger of SIS in recent years.

"If you have a strained knee and you don't allow that knee to recover, you are not only going to delay that recovery, but you're also in much greater danger for damaging it worse and possibly permanently," Gioia said. "That's the same concept with the brain."

"The No. 1 danger is that second hit, and that second hit we can absolutely prevent if everybody is on board with what we need to do."

The most famous case of SIS in the state occurred when Zackery Lystedt of Maple Valley suffered a life-threatening brain injury after going back into a football game following a head injury in 2006.

Former Sequim football player Adrien Gault suffered a similar fate a week later.

Both still feel the effects of their injury, with Lystedt unable to stand on his own two feet, with assistance, until three years afterward.

That same year the state Legislature unanimously passed the Lystedt Law in May 2009, prohibiting youth athletes suspected of sustaining a concussion from returning to play without a licensed health-care provider's approval.

Yet there is still education that needs to be done, according to Dr. Scott Laker, Clinical Assistant Professor for the University of Washington Sports and Spine Physicians.

"Even with the law that passed last year, most of the athletes that come in [to the clinic] still don't understand how serious the injuries might be," Laker said.

"I still don't think the word is out enough for understanding how serious this can be for men and women."

Across genders

Part of that, of course, lies with the perception of the injury as male only.

Girls have actually been found to have a higher incidence of concussions in several sports, including soccer, lacrosse and basketball.

While there is no unified opinion as to why that is, there are a number of theories.

Some opinions offer that girls' necks generally might not be as strong as boys', making them much more susceptible to head trauma.

Some suggest that girls don't have the same corrective balance as boys, while others argue that girls might be more willing to discuss their symptoms than boys.

Both Gioia and Laker don't buy the latter.

"I guess the way I think of it now is, where there is smoke there is probably fire," Laker said. "When we really get down to numbers, there are a few studies that disagree, but most of the literature seems to point that gender plays a part."

Added Gioia, "If you've ever worked with female athletes at the middle to upper level, they don't ever want to come out.

"I say that because we are looking at some of the toughest soccer and lacrosse players in this area, and they are looking at you like, 'Don't take me out of this game.'"

Off to college

O'Neil, a junior at the time of her injury, would never play another soccer match for Sequim.

She had actually been cleared to play by a doctor in Sequim a few days after the injury, but opted for another opinion from Dr. Stanley Herring, Director of Spine Care at the University of Washington.

She still displayed symptoms that pointed to lingering effects of a concussion and didn't pass neuropsychological tests measuring her memory.

Thus, Dr. Herring recommended she not play.

"I think there are some misunderstandings about what constitutes a concussion and what doesn't," Herring said during a May 2009 interview. "A ding is not always a ding.

"Most sports concussions do not involve being knocked out, only 10 percent do. Some people have a headache. Others are dizzy. Others might have problems with memories. It's not a one-size-fits-all situation."

O'Neil ended up missing 46 days of school during the 2008-09 school year because of the aftereffects of her injury.

It wasn't until that spring that she finally got to participate in athletics as part of Sequim's track team.

Now a freshman at the University of Washington, she has few regrets.

"I think it was the right idea," said O'Neil, who was one of seven valedictorians of her graduating class at Sequim.

"I regret it happening that I got a concussion and I couldn't play, but I don't regret sitting out because I've heard all of the stories of different people who went back in and got totally brain damaged.